pkg-merge-upstream-to-downstreams: Pull updates from debian backports

We may want to run the CI pipeline to pull updates from a debian release
backports upstream. Extend this script to handle those branches.

Signed-off-by: Ariel D'Alessandro <ariel.dalessandro@collabora.com>

package-source-builder/overlay/usr/bin/apertis-pkg-merge-upstream-to-downstreams

@@ -50,6 +50,7 @@ def existing_upstream_branches(upstream: str):
     branches = [
         f'{upstream}-updates',
         f'{upstream}-proposed-updates',
+        f'{upstream}-backports',
         f'{upstream}-security',
         f'{upstream}',
     ]